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1. General Description

Hydrology is the scientific study of the properties, distribution, use and circulation of the water of the Earth and the atmosphere in all of its forms.

In a catchment, such as that of the Upper Parramatta River, the study of hydrology includes such things as precipitation, surface runoff, infiltration, stream bank erosion and flood impacts.

Dry weather or base flows are greater in the Darling Mills Creek, Hunts Creek sub-catchments which are in Hawkesbury Sandstone areas in the north-east of the main catchment than in the creeks that originate in the Wianamatta Shale to the west and the south.

2. Hydrological Characteristics

Monthly Flow Rates.

The Department of Land and Water Conservation (DLWC) operates two stream gauging stations in the catchment:

·           Toongabbie Creek at Briens Road, Northmead (1979 - present);

·           Parramatta River at Cumberland Hospital, Parramatta (7/2/1979 - 7/2/1992).

Fig C5.1 Mean Monthly Flow at Stream Gauging Stations in the Catchment

Flow regime is the long term pattern of high flows and low flows in creeks and rivers.

Generally there is very little flow in catchment creeks for the majority of the time, (Downes, Masters Thesis, 1998). 98% of total flow volume leaves the catchment in only 2% of the time. The average dry weather flow across the catchment is 1890 litres/hectare/day.

Run-off  and Infiltration.

Runoff is the movement of water across the land surface.

Infiltration is the passage of water into the ground through permeable soils.

Runoff and infiltration are natural processes. Humans however, in some areas have dramatically altered these – sometimes with devastating consequences

When the catchment was in its natural state much of the rainfall was absorbed into the ground to be used by vegetation – herbs, trees and shrubs. Rough ground, rock outcrops and vegetation acted as obstacles slowing down surface runoff. With increasing development in the catchment and the proliferation of impervious surfaces such as roads, pavements, car parks, driveways etc, runoff has increased and infiltration has consequently decreased. This has been compounded by the channelling of water into drains, stormwater pipes and gutters so that flow rates have increased.

Figure C5.2 Percentage (%) Surface Runoff on a variety of surfaces

Altered Flow Regimes.

Urbanisation and more recently redevelopment to higher density land uses in the Upper Parramatta River Catchment has dramatically altered the original flow regime. Stormwater runoff volumes are now much greater, are delivered to the creeks more quickly and pass through the catchment in a much shorter time.

The reduction of infiltration means there is less ground water feeding the creeks in dry periods between storm events. This results in flooding, creek bank erosion and sedimentation during storm events and inadequate base flow, barriers and elevated water temperatures during dry periods.

Floods In the Upper Parramatta River Catchment market pressures and several court decisions in the 1970’s have encouraged substantial development in areas subject to some degree of flooding risk, eg parts of Seven Hills and Toongabbie. In recent times there has been several dramatic increases of flooding in the Upper Parramatta River Catchment resulting in many millions of dollars damage to local property. With increased urbanisation the impact on our land and waterways has increased. Only about 30% of the natural land in the catchment is now available to soak up the rainwater and most of the trees which once intercepted and stored water have also gone from the system. As development of the catchment has expanded to the current 70% urbanisation, the natural tendency of local creeks to flood has been exacerbated. As a direct result of increasing urbanisation the 100 year flood level in Parramatta has been increased by 3 metres since 1950. The rise and fall of flood waters in the Parramatta River can be shown in a hydrograph, (the amount of water passing a point, its discharge, at a certain time). Natural and urban catchment conditions are compared in the figure below.

Hart Street, North Wentworthville, 1988

Fig C5.3 Natural and Urban Hydrographs for the UPRC

Fig C5.4 Peak Discharges for Flood Waters in the Parramatta River

Station	Date	Time	Discharge m3/s
Marsden Weir Parramatta River	30/4/88	midnight	800
	5/8/86	6pm	500
Briens Road	30/4/88	Midnight	275

Channel Geomorphology

Creek bank erosion and sedimentation are natural processes resulting from changes in creek channels. Unfortunately the changed runoff characteristics of the catchment means that erosion on land and in creeks is occurring at a greater rate. Increased erosion from building sites and other exposed surfaces means higher sediment loads are carried by stream. This combination of increased run off peaks, accelerated erosion and increased sediment deposition causes unstable creek systems. Creek instability can undermine private property and infrastructure, and impact on environmental values through the loss of creek bank vegetation, poor water quality and loss of amenity. This problem is particularly severe in Darling Mills Creek where large deposits of sand from urban run off have filled in most waterholes and forced the creek to migrate resulting in erosion of banks.

3.Other Resources and Links

·           Upper Parramatta River Catchment Trust www.uprct.nsw.gov.au

·           CRC for catchment Hydrology www.catchment.crc.org.au

·           EPA www.epa.nsw.gov.au